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Carbon monoxide sensing at room temperature via electron donation in boron doped diamond films
Sensors and Actuators B 145 (2010) 527–532
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Carbon monoxide sensing at room temperature via electron donation in boron doped diamond films Rakesh K. Joshi a,∗ , Jessica E. Weber a , Qiang Hu a , Bob Johnson b , Jerry W. Zimmer b , Ashok Kumar a,∗ a b
Department of Mechanical Engineering, University of South Florida, 4202 E. Fowler Ave, Tampa, FL 33620, United States sp3 Diamond Technologies, 2220 Martin Ave., Santa Clara, CA 95050, United States
a r t i c l e
i n f o
Article history: Received 20 August 2009 Received in revised form 29 December 2009 Accepted 30 December 2009 Available online 13 January 2010 Keywords: Boron doped diamond Gas sensors
a b s t r a c t We report room temperature detection of carbon monoxide (CO) below 100 parts per million in air using boron doped diamond films prepared by hot filament chemical vapor deposition method. Gas sensing characteristics are observed to be improved with larger grain size in the films. Sensing characteristics for 100 ppm CO with response time of 30 s and recovery times of ∼50 s at room temperature show the excellence of boron doped diamond as CO sensor material. The lowest values of response time ∼5 s and recovery time ∼20 s were observed for BDD with grain size of 1 m for 1000 ppm of CO in air. The carbon monoxide sensing mechanism is attributed to electron donation to p-type boron doped diamond as a result of CO oxidation on its surface. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Carbon monoxide (CO) sensing has always been the subject of prime interest. The ambient air quality standard requires CO to be at a concentration lower than 100 parts per million (ppm). Progressive growth in the field of gas sensor technology requires minimal complexity involved in the fabrication technique. Several types of CO sensors, based on varied physical and chemical properties, have been suggested ranging from metal-oxide gas sensors to pure metallic sensors [1–5]. It has been a difficult task to detect CO at room temperature even after adding a metal catalyst into the semiconductor materials. Oxidation of CO by conductive materials such at Pt, Pd, Au and Ag [6–9] at room temperature and at very low temperatures (below 100 K) has been reported in the past which suggests that a highly conductive material can be used for room temperature CO sensors. In the present article we focus on studying the CO sensing characteristics of highly conductive boron doped diamond (BDD). We aim to identify the physical phenomenon responsible for CO gas sensing by BDD films. The idea underlying the selection of BDD was that it has variable conductivity, which makes it available for electron transport phenomenon in the presence of oxidizing and reducing gases. Due to its small size, the boron atom is able to be incorporated into the diamond lattice and offers very high conductivity to the film [10–13]. This feature of high conductivity suggests
∗ Corresponding authors. E-mail addresses: [email protected] (R.K. Joshi), [email protected] (A. Kumar). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.12.070
the potential to use BDD as conductivity based chemical gas sensor and opens a broad area of research for new physio-chemical gas sensor phenomenon on conductive diamond surfaces. This article reports the detection of 100 ppm CO in air at room temperature and explains in detail the gas sensing phenomena based on basic gas adsorption chemistry on the surface of conductive boron doped diamond films. 2. Experimental Boron doped diamond films on silicon are deposited with a hot filament CVD reactor (sp3 Diamond Technologies, Inc. Model 650); capable of depositing on one 300 mm or nine 100 mm wafers in the large industrial chamber with wafer film thickness uniformity better than ±5% and wafer-to-wafer uniformity of typically ±3% for a nine-wafers run. The silicon wafers are seeded prior to deposition in an ultrasonically agitated solution containing 4–50 nm ultra dispersed diamond (UDD) detonation-synthesized nanopowder. The high seed density (>5 × 1010 /cm2 ) allows these films to be characterized as pinhole free with uniform grain morphology. The grain size of the diamond is varied from large grain microcrystalline to small grain nanocrystalline by controlling the CH4 and pressure set points to the required process window. Boron is introduced into the system as the trimethylborate (TMB) gas to produce p-type doping. Increasing the TMB flow during deposition allows more boron to be incorporated into the diamond film to reduce the sheet resistance. Tungsten wires were heated to ∼2200 ◦ C to activate the reaction gas—a mixture of hydrogen and methane. Film parameters with growth conditions for two BDD films with highest and lowest grain size are shown in Table 1. All the films used for the present study
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Fig. 1. Schematic diagram of gas sensing measurement setup.
Table 1 Properties and process parameters for diamond films with microcrystalline and nanocrystalline nature. Average grain size Sheet resistance Resistivity Watt density for growth %CH4 for growth %TMB for growth Wire temp for growth Substrate temperature
1 m 23 /sq 0.019 cm 11.2 W/cm2 1.5% 0.007% 2270 ◦ C 800 ◦ C
80 nm 1.0 k/sq 0.32 cm 9.6 W/cm2 2.2% 0.007% 2255 ◦ C 825 ◦ C
are nearly with same thickness of ∼1 m. Boron doped diamond films with the highest average grain size (1 m) are characterized as mBDD and the films with lowest average grain size (80 nm) are characterized as nmBDD. Grain sizes were calculated using the
atomic force microscopy and X-ray diffraction peak broadening. These values were in agreement with grain size values obtained using transmission electron microscopy of few selected samples. The HFCVD system used to grow the diamond films for the present study has high degree of reproducibility in surface morphology and grain sizes. The BDD films were tested for gas sensing behavior using four probe resistance measurements in the presence of compressed air and carbon monoxide using an electrometer (Keithley 2400) and a gas controller (MKS 247). A sample heater with Eurotherm-2416 temperature controller supplied from Blue Wave Semiconductors Inc. was used to perform high temperature gas sensor measurements. CO in concentrations of 100 and 1000 ppm mixed in compressed air was purchased from Airgas, Inc., whereas 500 ppm CO concentration was obtained by making a proper dilution of 1000 ppm in compressed air using the mass flow con-
Fig. 2. Atomic force microscopy and Raman spectroscopy for mBDD (1 m grain size) and nmBDD (80 nm grain size) films.
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trollers. The gas sensors were characterized by the sensor signal which is defined as percent change in resistance of the film upon CO exposure. If Ra is the resistance in air and Rb is the resistance in the presence of CO then the sensor signal is defined as [((Ra − Rb )/Ra ) × 100]. Fig. 1 shows the schematic diagram of the gas sensor setup used for this study. Ohmic electrical contacts were obtained using spring loaded pogo pins with gold at the tip. This arrangement helps in reducing the noise level during the resistance measurements. The BDD films show very stable resistance values during all the set of measurements.
3. Results and discussion Surface morphology for the BDD films was studied using atomic force microscope and structure of the films was studied by Raman spectroscopy. Atomic force microscopy (AFM) and Raman spectroscopy for mBDD (1 m grain size) and nmBDD (80 nm grain size) films are shown in Fig. 2. Raman spectra for high quality diamond and boron doped diamond films feature one intense band at 1332 cm−1 with FWHM ∼10 cm−1 . A wide band arises around 1220 cm−1 which indicated the presence of boron. In Raman spectrum, the band corresponding to boron is narrower for mBDD as compared to the nmBDD which is probably due to the larger grains in microcrystalline diamond [14]. The Raman spectra for nanocrystalline diamond films shows a large disorder band due to impurity whereas the microcrystalline diamond shows pure diamond nature. The boron doped diamond films exhibited an increase in resistance upon CO exposure for all films. Fig. 3a shows a typical sensor response which is defined as percent change in resistance (−R/R0 ) of the BDD films upon CO exposure as a function of time. The gas sensing characteristics were observed to improve with an increase in grain size from 80 nm (nmBDD) to 1 m grain size (mBDD). Fig. 2b shows the variation of sensor signal with average grain size of diamond particles in BDD films for different concentrations of carbon monoxide. Sensing results are based on the change in resistance of the BDD sensor on CO exposure using four probe measurements. The measurements were repeated three times for the same samples with different electrical contact points and the same conditions. Reproducibility of the sensors was testified on the samples grown under the same conditions. Sensor characteristics were observed to be nearly same at all time. The error bars in Fig. 3b show the deviation from the average value of the sensor signal on repeating the measurements for sensors with same film properties but prepared separately in HFCVD. It was observed for all the CO concentrations that the diamond films with larger grain size are more sensitive than the films with smaller grains. This is due to the high conductive nature of diamond films with higher grain size. High conductivity is required for the room temperature gas sensing. The mBDD films are nearly metallic in nature with very low resistivity (∼0.02 cm) while the nmBDD behave as a highly conductive-semiconductor with resistivity of 0.32 cm. Therefore, at room temperature, the nmBDD is not sufficiently activated to react with CO and cause a change in resistance needed to produce the sensor signal. In other words, the number of activated free carriers decreases as particle size decreases, with the lowest value for nmBDD (80 nm grain size) films. According to the size dependent gas sensing behavior of nanocrystalline materials the nmBDD should show better gas sensing than the mBDD diamond due to higher gas adsorption at the active centers on the surface of diamond films caused by the enhanced surface states at the nanometer scale. The proposed mechanism explains the decrease of conductance on CO exposure. The schematic in Fig. 4 shows various steps of CO sensing by BDD films. The conductance decrease, which is rep-
Fig. 3. (a) Variation of sensor signal with time on repeatedly switching from compressed air to 100 ppm of carbon monoxide for boron doped diamond films with different grain size. (b) Variation of sensor signal with average grain size in boron doped diamond films for different concentrations of carbon monoxide at room temperature.
resented as sensor signal, is higher in mBDD than nmBDD even though the nmBDD has more active centers for gas adsorption due to a higher number of grain boundaries in nanocrystalline films. Huang et al., have reported that both the diamond grain boundaries as well as the crystallite surfaces are the sources which possess active centers for oxygenation and adsorption of other gases [15]. Atomic force microscope images show the grains and grain boundaries (Fig. 2). In order to explain the CO sensing mechanism we selected a portion of the AFM micrographs which represents an average for respective mBDD (highest grain size) and nmBDD (lowest grain size) films. Carbon monoxide is an electron donating reducing gas with CO molecules adsorbed onto conductive diamond surfaces. Grain boundaries in the diamond films are active sites (AS) or the most probable sites for the gas adsorption. The active sites are marked by yellow arrows in the schematic shown in Fig. 4. The sensors were first exposed to a continuous flow of air (O2 ) which reacts on conductive diamond surface, micro as well as nano, and dissociates into O− . The dissociated species adsorbed primarily on grain boundaries in P-type boron doped diamond. During the resistance measurement, after switching gas from O2 to CO, the CO molecules also get adsorbed onto the conductive diamond surface and interact with the pre-adsorbed O− via the well-known oxidation reaction [16,17] CO + O− = CO2 (g) + e−
(1)
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The CO oxidation during the continuous flow leads to the transfer of electrons from O− to the p-type semiconducting BDD films and initiates charge neutrality. This process causes depletion in the total number of charge carriers and leads to a decrease in the conductance due to the unavailability of free carriers for conduction by boron doped diamond. As the average grain size decreases from micrometer to nanometer range the charge carrier density decreases in semiconductors due to enhanced grain boundaries and surface states [18,19]. The available free carriers (holes) are less abundant in nmBDD than the mBDD to combine the electrons released from the oxidation according to Eq. (1). Clearly, the consumption of electrons by mBDD is more than the consumption of electrons by nmBDD. As an outcome of this process we observe greater decrease in conductance, or greater sensor signal for mBDD rather than the nmBDD in the presence of CO exposure. This feature is depicted in Fig. 4 by showing some free electrons in nmBDD available for conduction even after some of them have been captured by the nmBDD p-type semiconductor. In the equilibrium state, the rate of CO oxidation, which is the rate of electron donation, becomes constant and the conductance decrease obtains the saturation as shown in the response plot (Fig. 3a). Response and recovery times for the sensors, as expected, are lower for higher concentration of the gas. The response and recovery times are important factors to assess the performance of a gas sensor. The response time is defined as the time needed for the resistance of the gas sensor to obtain 90% of the maximum conductance when CO gas is introduced into an environment of
Fig. 4. Schematic for different steps (1–3) of CO sensing by boron doped micro/nanocrystalline diamond films. O− is represented by white open circle and CO molecule is represented as . The transfer of the electron (䊉) (captured by O− ) to the p-type semiconducting BDD films with carriers as hole () is shown in step 3. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of the article.)
Table 2 Response and recovery time for mBDD and nmBDD films for different concentrations for CO at room temperature. CO concentration (ppm)
1000 500 100 10
Response time (s)
Recovery time (s)
mBDD
nmBDD
mBDD
nmBDD
5 20 30 60
25 40 65 95
20 45 50 110
55 100 140 210
air. The recovery time is the time required for 90% reduction in resistance when CO is turned off [20,21]. The low value of response time (∼30 s) and recovery time (∼50 s) for 100 ppm of CO at room temperature using BDD films with 1 m grain size suggests the high potential of boron doped diamond in gas sensor technology. However, the lowest values of response time as low as 5 s and recovery time as low as 20 s were observed for BDD with grain size of 1 m for 1000 ppm of CO in air. Table 2 shows the values of response and recovery times for the films at room temperature. In this context, BDD can be added into the class of materials which exhibit CO oxidation at room temperature. Fig. 5 shows the CO concentration dependence of the gas sensor signal for as-grown and annealed BDD based gas sensors. All the samples were annealed at 300 ◦ C in argon atmosphere for three hours. Annealing of the boron doped samples in inert atmosphere results into a very small loss of conductivity of the samples reflected in terms of the increase of resistance of the samples measured by four probe methods. Effect of annealing on electronic properties of diamond films has been extensively studied in the past [22–25]. The annealed samples show slight improvement in sensor signal values. However, the improvement in sensor performance of the annealed sample is not enough to suggest any mechanism responsible for this behavior. Error bars in plot show the deviation from the average value of the sensor signal estimated from three sets of measurements. In order to improve the sensing characteristics of the boron doped diamond films we tested the sensors in various conditions. Boron doped diamond based sensors were tested at higher temperature to avoid moisture effect on the surface. Fig. 6a and b shows the variation of sensor signal with temperature for mBDD and nmBDD sensors. Temperature effect on sensing behavior is higher in the case of nmBDD than in the case of mBDD sensors for all concentrations of gases. Increase in sensor signal with temperature
Fig. 5. Variation of gas sensor signal with CO concentration for mBDD and nmBDD as-grown and annealed (300 ◦ C) films.
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References
Fig. 6. Variation of gas sensor signal with operating temperature for mBDD (a) and nmBDD (b) films.
suggests enhancement in gas adsorption at higher temperature. Since the nanocrystalline diamond has more grain boundaries compared to the microcrystalline diamond, the temperature effect is higher in the case of nanocrystalline diamond based gas sensors. The observed temperature variation of sensor performance indicates the grain boundary dependent gas sensors mechanism [26] in the films. 4. Conclusions Gas sensing characteristics of B doped conductive diamond films were examined for different concentrations of carbon monoxide in air at room temperature. Experimentally observed response time of ∼30 s and recovery time of ∼50 s for 100 ppm of CO at room temperature indicates high potential of boron doped diamond in gas sensor technology. CO gas sensing in the present case is attributed to the electron donation to p-type boron doped diamond as a result of CO oxidation on its surface.
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Biographies
Acknowledgements
Rakesh K. Joshi received the PhD degree in physics from the Indian Institute of Technology, Delhi. He worked as a Postdoctoral Fellow at the University of Duisburg, Duisburg, Germany, and the Nano High Research Center, Toyota Technological Institute, Nagoya, Japan. He has published over 35 journal articles in the field of thin films, nanomaterials and sensors. He is an associate editor for the Journal of Nanomaterials (JNM) and edited three special issues for JNM including one in the Nanosensor Technology. Dr Joshi is currently with Mechanical Engineering department of the University of South Florida.
This work was supported by National Science Foundation through NIRT # ECS 0404137, IGERT #0221681 and GK12 #0638709, as well as by the GMS University of South Florida thrust initiative GFMMD00.
Jessica E. Weber is a PhD candidate in mechanical engineering at the University of South Florida and is expected to graduate in spring 2010. She received her BSME in 2003 and MSME in 2005 from USF as well. During her graduate career, Ms. Weber has been the recipient of NSF IGERT (2004–2006) and NSF GK-12 (2007–present) Fellowships. Her research focus is on functional nanomaterials with an electrochemistry-based approach to sensing and energy applications.
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Qiang Hu is a PhD candidate in mechanical engineering at the University of South Florida and is expected to graduate in spring 2010. His research focus is on carbon based nanomaterials for sensor and actuators. Bob Johnson is executive vice president sales and marketing at Seki Technotron USA. He has vast experience in semiconductor processing diamond deposition. He is the author of several patents and has authored numerous papers on diamond technology. Jerry W. Zimmer is chief technical officer and co-founder of sp3 Inc. He currently directs all process, product and equipment development activities at sp3 and was the primary architect of their hot filament diamond equipment and deposition technology. He has more than 20 years of semiconductor processing experience and 19 years
of diamond deposition experience. He is the author or co-author of fifteen patents and has authored or co-authored more than a dozen papers on diamond technology, automation technology and semiconductor thin film deposition processes among other topics. Ashok Kumar received the PhD degree in materials science and engineering from North Carolina State University, Raleigh. Prof. Kumar is the director of the Nanomaterials and Nanomanufacturing Research Center and professor with the Department of Mechanical Engineering, University of South Florida. He has published over 100 research articles, 5 book chapters, and has several patents on his name. He has edited four books in the field of materials science and nanotechnology. Prof. Kumar is a recipient of the NSF Faculty Early Career Development Award and ASM-IIM Visiting Lecture Award. He is a Fellow of American Society of Metals.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Carbon monoxide sensing at room temperature via electron donation in boron doped diamond films Rakesh K. Joshi a,∗ , Jessica E. Weber a , Qiang Hu a , Bob Johnson b , Jerry W. Zimmer b , Ashok Kumar a,∗ a b
Department of Mechanical Engineering, University of South Florida, 4202 E. Fowler Ave, Tampa, FL 33620, United States sp3 Diamond Technologies, 2220 Martin Ave., Santa Clara, CA 95050, United States
a r t i c l e
i n f o
Article history: Received 20 August 2009 Received in revised form 29 December 2009 Accepted 30 December 2009 Available online 13 January 2010 Keywords: Boron doped diamond Gas sensors
a b s t r a c t We report room temperature detection of carbon monoxide (CO) below 100 parts per million in air using boron doped diamond films prepared by hot filament chemical vapor deposition method. Gas sensing characteristics are observed to be improved with larger grain size in the films. Sensing characteristics for 100 ppm CO with response time of 30 s and recovery times of ∼50 s at room temperature show the excellence of boron doped diamond as CO sensor material. The lowest values of response time ∼5 s and recovery time ∼20 s were observed for BDD with grain size of 1 m for 1000 ppm of CO in air. The carbon monoxide sensing mechanism is attributed to electron donation to p-type boron doped diamond as a result of CO oxidation on its surface. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Carbon monoxide (CO) sensing has always been the subject of prime interest. The ambient air quality standard requires CO to be at a concentration lower than 100 parts per million (ppm). Progressive growth in the field of gas sensor technology requires minimal complexity involved in the fabrication technique. Several types of CO sensors, based on varied physical and chemical properties, have been suggested ranging from metal-oxide gas sensors to pure metallic sensors [1–5]. It has been a difficult task to detect CO at room temperature even after adding a metal catalyst into the semiconductor materials. Oxidation of CO by conductive materials such at Pt, Pd, Au and Ag [6–9] at room temperature and at very low temperatures (below 100 K) has been reported in the past which suggests that a highly conductive material can be used for room temperature CO sensors. In the present article we focus on studying the CO sensing characteristics of highly conductive boron doped diamond (BDD). We aim to identify the physical phenomenon responsible for CO gas sensing by BDD films. The idea underlying the selection of BDD was that it has variable conductivity, which makes it available for electron transport phenomenon in the presence of oxidizing and reducing gases. Due to its small size, the boron atom is able to be incorporated into the diamond lattice and offers very high conductivity to the film [10–13]. This feature of high conductivity suggests
∗ Corresponding authors. E-mail addresses: [email protected] (R.K. Joshi), [email protected] (A. Kumar). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.12.070
the potential to use BDD as conductivity based chemical gas sensor and opens a broad area of research for new physio-chemical gas sensor phenomenon on conductive diamond surfaces. This article reports the detection of 100 ppm CO in air at room temperature and explains in detail the gas sensing phenomena based on basic gas adsorption chemistry on the surface of conductive boron doped diamond films. 2. Experimental Boron doped diamond films on silicon are deposited with a hot filament CVD reactor (sp3 Diamond Technologies, Inc. Model 650); capable of depositing on one 300 mm or nine 100 mm wafers in the large industrial chamber with wafer film thickness uniformity better than ±5% and wafer-to-wafer uniformity of typically ±3% for a nine-wafers run. The silicon wafers are seeded prior to deposition in an ultrasonically agitated solution containing 4–50 nm ultra dispersed diamond (UDD) detonation-synthesized nanopowder. The high seed density (>5 × 1010 /cm2 ) allows these films to be characterized as pinhole free with uniform grain morphology. The grain size of the diamond is varied from large grain microcrystalline to small grain nanocrystalline by controlling the CH4 and pressure set points to the required process window. Boron is introduced into the system as the trimethylborate (TMB) gas to produce p-type doping. Increasing the TMB flow during deposition allows more boron to be incorporated into the diamond film to reduce the sheet resistance. Tungsten wires were heated to ∼2200 ◦ C to activate the reaction gas—a mixture of hydrogen and methane. Film parameters with growth conditions for two BDD films with highest and lowest grain size are shown in Table 1. All the films used for the present study
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Fig. 1. Schematic diagram of gas sensing measurement setup.
Table 1 Properties and process parameters for diamond films with microcrystalline and nanocrystalline nature. Average grain size Sheet resistance Resistivity Watt density for growth %CH4 for growth %TMB for growth Wire temp for growth Substrate temperature
1 m 23 /sq 0.019 cm 11.2 W/cm2 1.5% 0.007% 2270 ◦ C 800 ◦ C
80 nm 1.0 k/sq 0.32 cm 9.6 W/cm2 2.2% 0.007% 2255 ◦ C 825 ◦ C
are nearly with same thickness of ∼1 m. Boron doped diamond films with the highest average grain size (1 m) are characterized as mBDD and the films with lowest average grain size (80 nm) are characterized as nmBDD. Grain sizes were calculated using the
atomic force microscopy and X-ray diffraction peak broadening. These values were in agreement with grain size values obtained using transmission electron microscopy of few selected samples. The HFCVD system used to grow the diamond films for the present study has high degree of reproducibility in surface morphology and grain sizes. The BDD films were tested for gas sensing behavior using four probe resistance measurements in the presence of compressed air and carbon monoxide using an electrometer (Keithley 2400) and a gas controller (MKS 247). A sample heater with Eurotherm-2416 temperature controller supplied from Blue Wave Semiconductors Inc. was used to perform high temperature gas sensor measurements. CO in concentrations of 100 and 1000 ppm mixed in compressed air was purchased from Airgas, Inc., whereas 500 ppm CO concentration was obtained by making a proper dilution of 1000 ppm in compressed air using the mass flow con-
Fig. 2. Atomic force microscopy and Raman spectroscopy for mBDD (1 m grain size) and nmBDD (80 nm grain size) films.
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trollers. The gas sensors were characterized by the sensor signal which is defined as percent change in resistance of the film upon CO exposure. If Ra is the resistance in air and Rb is the resistance in the presence of CO then the sensor signal is defined as [((Ra − Rb )/Ra ) × 100]. Fig. 1 shows the schematic diagram of the gas sensor setup used for this study. Ohmic electrical contacts were obtained using spring loaded pogo pins with gold at the tip. This arrangement helps in reducing the noise level during the resistance measurements. The BDD films show very stable resistance values during all the set of measurements.
3. Results and discussion Surface morphology for the BDD films was studied using atomic force microscope and structure of the films was studied by Raman spectroscopy. Atomic force microscopy (AFM) and Raman spectroscopy for mBDD (1 m grain size) and nmBDD (80 nm grain size) films are shown in Fig. 2. Raman spectra for high quality diamond and boron doped diamond films feature one intense band at 1332 cm−1 with FWHM ∼10 cm−1 . A wide band arises around 1220 cm−1 which indicated the presence of boron. In Raman spectrum, the band corresponding to boron is narrower for mBDD as compared to the nmBDD which is probably due to the larger grains in microcrystalline diamond [14]. The Raman spectra for nanocrystalline diamond films shows a large disorder band due to impurity whereas the microcrystalline diamond shows pure diamond nature. The boron doped diamond films exhibited an increase in resistance upon CO exposure for all films. Fig. 3a shows a typical sensor response which is defined as percent change in resistance (−R/R0 ) of the BDD films upon CO exposure as a function of time. The gas sensing characteristics were observed to improve with an increase in grain size from 80 nm (nmBDD) to 1 m grain size (mBDD). Fig. 2b shows the variation of sensor signal with average grain size of diamond particles in BDD films for different concentrations of carbon monoxide. Sensing results are based on the change in resistance of the BDD sensor on CO exposure using four probe measurements. The measurements were repeated three times for the same samples with different electrical contact points and the same conditions. Reproducibility of the sensors was testified on the samples grown under the same conditions. Sensor characteristics were observed to be nearly same at all time. The error bars in Fig. 3b show the deviation from the average value of the sensor signal on repeating the measurements for sensors with same film properties but prepared separately in HFCVD. It was observed for all the CO concentrations that the diamond films with larger grain size are more sensitive than the films with smaller grains. This is due to the high conductive nature of diamond films with higher grain size. High conductivity is required for the room temperature gas sensing. The mBDD films are nearly metallic in nature with very low resistivity (∼0.02 cm) while the nmBDD behave as a highly conductive-semiconductor with resistivity of 0.32 cm. Therefore, at room temperature, the nmBDD is not sufficiently activated to react with CO and cause a change in resistance needed to produce the sensor signal. In other words, the number of activated free carriers decreases as particle size decreases, with the lowest value for nmBDD (80 nm grain size) films. According to the size dependent gas sensing behavior of nanocrystalline materials the nmBDD should show better gas sensing than the mBDD diamond due to higher gas adsorption at the active centers on the surface of diamond films caused by the enhanced surface states at the nanometer scale. The proposed mechanism explains the decrease of conductance on CO exposure. The schematic in Fig. 4 shows various steps of CO sensing by BDD films. The conductance decrease, which is rep-
Fig. 3. (a) Variation of sensor signal with time on repeatedly switching from compressed air to 100 ppm of carbon monoxide for boron doped diamond films with different grain size. (b) Variation of sensor signal with average grain size in boron doped diamond films for different concentrations of carbon monoxide at room temperature.
resented as sensor signal, is higher in mBDD than nmBDD even though the nmBDD has more active centers for gas adsorption due to a higher number of grain boundaries in nanocrystalline films. Huang et al., have reported that both the diamond grain boundaries as well as the crystallite surfaces are the sources which possess active centers for oxygenation and adsorption of other gases [15]. Atomic force microscope images show the grains and grain boundaries (Fig. 2). In order to explain the CO sensing mechanism we selected a portion of the AFM micrographs which represents an average for respective mBDD (highest grain size) and nmBDD (lowest grain size) films. Carbon monoxide is an electron donating reducing gas with CO molecules adsorbed onto conductive diamond surfaces. Grain boundaries in the diamond films are active sites (AS) or the most probable sites for the gas adsorption. The active sites are marked by yellow arrows in the schematic shown in Fig. 4. The sensors were first exposed to a continuous flow of air (O2 ) which reacts on conductive diamond surface, micro as well as nano, and dissociates into O− . The dissociated species adsorbed primarily on grain boundaries in P-type boron doped diamond. During the resistance measurement, after switching gas from O2 to CO, the CO molecules also get adsorbed onto the conductive diamond surface and interact with the pre-adsorbed O− via the well-known oxidation reaction [16,17] CO + O− = CO2 (g) + e−
(1)
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The CO oxidation during the continuous flow leads to the transfer of electrons from O− to the p-type semiconducting BDD films and initiates charge neutrality. This process causes depletion in the total number of charge carriers and leads to a decrease in the conductance due to the unavailability of free carriers for conduction by boron doped diamond. As the average grain size decreases from micrometer to nanometer range the charge carrier density decreases in semiconductors due to enhanced grain boundaries and surface states [18,19]. The available free carriers (holes) are less abundant in nmBDD than the mBDD to combine the electrons released from the oxidation according to Eq. (1). Clearly, the consumption of electrons by mBDD is more than the consumption of electrons by nmBDD. As an outcome of this process we observe greater decrease in conductance, or greater sensor signal for mBDD rather than the nmBDD in the presence of CO exposure. This feature is depicted in Fig. 4 by showing some free electrons in nmBDD available for conduction even after some of them have been captured by the nmBDD p-type semiconductor. In the equilibrium state, the rate of CO oxidation, which is the rate of electron donation, becomes constant and the conductance decrease obtains the saturation as shown in the response plot (Fig. 3a). Response and recovery times for the sensors, as expected, are lower for higher concentration of the gas. The response and recovery times are important factors to assess the performance of a gas sensor. The response time is defined as the time needed for the resistance of the gas sensor to obtain 90% of the maximum conductance when CO gas is introduced into an environment of
Fig. 4. Schematic for different steps (1–3) of CO sensing by boron doped micro/nanocrystalline diamond films. O− is represented by white open circle and CO molecule is represented as . The transfer of the electron (䊉) (captured by O− ) to the p-type semiconducting BDD films with carriers as hole () is shown in step 3. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of the article.)
Table 2 Response and recovery time for mBDD and nmBDD films for different concentrations for CO at room temperature. CO concentration (ppm)
1000 500 100 10
Response time (s)
Recovery time (s)
mBDD
nmBDD
mBDD
nmBDD
5 20 30 60
25 40 65 95
20 45 50 110
55 100 140 210
air. The recovery time is the time required for 90% reduction in resistance when CO is turned off [20,21]. The low value of response time (∼30 s) and recovery time (∼50 s) for 100 ppm of CO at room temperature using BDD films with 1 m grain size suggests the high potential of boron doped diamond in gas sensor technology. However, the lowest values of response time as low as 5 s and recovery time as low as 20 s were observed for BDD with grain size of 1 m for 1000 ppm of CO in air. Table 2 shows the values of response and recovery times for the films at room temperature. In this context, BDD can be added into the class of materials which exhibit CO oxidation at room temperature. Fig. 5 shows the CO concentration dependence of the gas sensor signal for as-grown and annealed BDD based gas sensors. All the samples were annealed at 300 ◦ C in argon atmosphere for three hours. Annealing of the boron doped samples in inert atmosphere results into a very small loss of conductivity of the samples reflected in terms of the increase of resistance of the samples measured by four probe methods. Effect of annealing on electronic properties of diamond films has been extensively studied in the past [22–25]. The annealed samples show slight improvement in sensor signal values. However, the improvement in sensor performance of the annealed sample is not enough to suggest any mechanism responsible for this behavior. Error bars in plot show the deviation from the average value of the sensor signal estimated from three sets of measurements. In order to improve the sensing characteristics of the boron doped diamond films we tested the sensors in various conditions. Boron doped diamond based sensors were tested at higher temperature to avoid moisture effect on the surface. Fig. 6a and b shows the variation of sensor signal with temperature for mBDD and nmBDD sensors. Temperature effect on sensing behavior is higher in the case of nmBDD than in the case of mBDD sensors for all concentrations of gases. Increase in sensor signal with temperature
Fig. 5. Variation of gas sensor signal with CO concentration for mBDD and nmBDD as-grown and annealed (300 ◦ C) films.
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References
Fig. 6. Variation of gas sensor signal with operating temperature for mBDD (a) and nmBDD (b) films.
suggests enhancement in gas adsorption at higher temperature. Since the nanocrystalline diamond has more grain boundaries compared to the microcrystalline diamond, the temperature effect is higher in the case of nanocrystalline diamond based gas sensors. The observed temperature variation of sensor performance indicates the grain boundary dependent gas sensors mechanism [26] in the films. 4. Conclusions Gas sensing characteristics of B doped conductive diamond films were examined for different concentrations of carbon monoxide in air at room temperature. Experimentally observed response time of ∼30 s and recovery time of ∼50 s for 100 ppm of CO at room temperature indicates high potential of boron doped diamond in gas sensor technology. CO gas sensing in the present case is attributed to the electron donation to p-type boron doped diamond as a result of CO oxidation on its surface.
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Biographies
Acknowledgements
Rakesh K. Joshi received the PhD degree in physics from the Indian Institute of Technology, Delhi. He worked as a Postdoctoral Fellow at the University of Duisburg, Duisburg, Germany, and the Nano High Research Center, Toyota Technological Institute, Nagoya, Japan. He has published over 35 journal articles in the field of thin films, nanomaterials and sensors. He is an associate editor for the Journal of Nanomaterials (JNM) and edited three special issues for JNM including one in the Nanosensor Technology. Dr Joshi is currently with Mechanical Engineering department of the University of South Florida.
This work was supported by National Science Foundation through NIRT # ECS 0404137, IGERT #0221681 and GK12 #0638709, as well as by the GMS University of South Florida thrust initiative GFMMD00.
Jessica E. Weber is a PhD candidate in mechanical engineering at the University of South Florida and is expected to graduate in spring 2010. She received her BSME in 2003 and MSME in 2005 from USF as well. During her graduate career, Ms. Weber has been the recipient of NSF IGERT (2004–2006) and NSF GK-12 (2007–present) Fellowships. Her research focus is on functional nanomaterials with an electrochemistry-based approach to sensing and energy applications.
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Qiang Hu is a PhD candidate in mechanical engineering at the University of South Florida and is expected to graduate in spring 2010. His research focus is on carbon based nanomaterials for sensor and actuators. Bob Johnson is executive vice president sales and marketing at Seki Technotron USA. He has vast experience in semiconductor processing diamond deposition. He is the author of several patents and has authored numerous papers on diamond technology. Jerry W. Zimmer is chief technical officer and co-founder of sp3 Inc. He currently directs all process, product and equipment development activities at sp3 and was the primary architect of their hot filament diamond equipment and deposition technology. He has more than 20 years of semiconductor processing experience and 19 years
of diamond deposition experience. He is the author or co-author of fifteen patents and has authored or co-authored more than a dozen papers on diamond technology, automation technology and semiconductor thin film deposition processes among other topics. Ashok Kumar received the PhD degree in materials science and engineering from North Carolina State University, Raleigh. Prof. Kumar is the director of the Nanomaterials and Nanomanufacturing Research Center and professor with the Department of Mechanical Engineering, University of South Florida. He has published over 100 research articles, 5 book chapters, and has several patents on his name. He has edited four books in the field of materials science and nanotechnology. Prof. Kumar is a recipient of the NSF Faculty Early Career Development Award and ASM-IIM Visiting Lecture Award. He is a Fellow of American Society of Metals.